1. Field of the Invention
The present invention relates to a fuel gas manufacturing apparatus for manufacturing a hydrogen-rich fuel gas by modifying a hydrogen-containing fuel which contains hydrocarbon or alcohol, and a method of operating such a fuel gas manufacturing apparatus.
2. Description of the Related Art
There has been employed in the art a hydrogen manufacturing apparatus (fuel gas manufacturing apparatus) for modifying a hydrocarbon fuel such as natural gas or the like or a hydrogen-containing fuel containing alcohol such as methanol or the like to produce a hydrogen-containing gas (modified gas) and supplying the hydrogen-containing gas as a fuel gas to a fuel cell or the like.
Such a hydrogen manufacturing apparatus basically operates to modify a hydrocarbon fuel such as a natural gas, a city gas, or the like with water vapor to produce a hydrogen-containing gas as a high-concentration hydrogen-rich gas, and separate high-purity hydrogen from the hydrogen-containing gas with a PSA (Pressure Swing Adsorption) device according to pressure adsorption.
Specifically, a hydrogen-containing gas produced by modifying a hydrocarbon fuel with water vapor contains various unwanted materials including CO, CO2, H2O, CH4, etc. in addition to a hydrogen gas as a major component. The PSA device has three adsorption towers, for example, each cyclically operable in adsorption, depressurization, pressure uniformization, blowing-down, and purging processes for extracting high-purity hydrogen and discharging the other components or unwanted materials as an off gas.
The off gas discharged from the PSA device is usually temporarily stored in an off-gas tank and supplied therefrom as a combusting fuel to a burner (heater) of a hydrogen-manufacturing modifier. Since the off-gas tank is intermittently supplied with the off gas from each of the adsorption towers, the off-gas tank is subject to pressure fluctuations. Such pressure fluctuations in the off-gas tank adversely affect the operation of the modifier and the PSA device, and tend to change the combustion state in the burner. Therefore, the pressure fluctuations in the off-gas tank need to be reduced.
There is known in the art a method of controlling the pressure of an off gas as disclosed in Japanese Laid-Open Patent Publication No. 2001-10806, for example. According to the known method, as shown in FIG. 8 of the accompanying drawings, an off-gas tank 2 is connected to an off-gas passage 1 extending from a PSA device to a modifier combustor. The off-gas tank 2 has an outlet conduit connected to a pressure meter 3. An off-gas flow rate adjusting valve 4 is connected to the outlet conduit of the off-gas tank 2 downstream of the pressure meter 3. The opening of the off-gas flow rate adjusting valve 4 is variable depending on the actual pressure of the off gas measured by the pressure meter 3. A flow rate meter 5 is connected to the outlet conduit of the off-gas tank 2 downstream of the off-gas flow rate adjusting valve 4.
Using the minimum pressure in the off-gas tank 2 as a reference, the opening of the off-gas flow rate adjusting valve 4 is varied at small intervals up to its full opening level for reducing pressure fluctuations of the off gas from the off-gas tank 2 to stably supply the off gas to the burner of the modifier combustor.
However, the off-gas tank 2 is required to be of such a size several times greater than the PSA device for effectively performing its function, and hence the hydrogen manufacturing apparatus is of a considerably large size. In particular, the hydrogen manufacturing apparatus with the off-gas tank 2 cannot be used as a hydrogen manufacturing apparatus for household use because an installation space available therefor in household applications is usually small.